SPMC_ARR_BASESZ
#define TEST_SPMC_RESIZE_PREFILL ((SPMC_ARR_BASESZ << TEST_SPMC_RESIZE_ORDER) - 1)
#define TEST_SPMC_RESIZE_TAIL (SPMC_ARR_BASESZ << TEST_SPMC_RESIZE_ORDER)
return SPMC_ARR_BASESZ << spmc_arr->order;
arr->data = arena_malloc((SPMC_ARR_BASESZ << order) * sizeof(*arr->data));